import java.util.Stack;

public class Valid {
    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);

            if(ch == '(' || ch == '{' || ch == '[') {
                stack.push(ch);
            } else {
                //ch = 右括号
                if(stack.isEmpty()) {
                    return false;
                }
                //拿到栈顶元素
                char peekCh = stack.peek();
                if(peekCh == '(' && ch == ')'
                        || peekCh == '{' && ch == '}'
                        ||peekCh == '[' && ch == ']') {
                    stack.pop();
                } else {
                    return false;
                }
            }
        }
        if(!stack.isEmpty()) {
            return false;
        }
        return true;
    }
}
